1. Field of the Invention
The present invention generally relates to safety instrumentation for rotary wing aircraft and, more particularly, to a warning system to alert the pilot of a low-G (“weightless”) condition which, if not promptly corrected, can result in catastrophic failure of the aircraft.
2. Background Description
Rotary wing aircraft (i.e., helicopters) have a main rotor that has from two to four blades. Helicopters furthermore have a tail rotor or lateral force generator which provides a thrust counter to the counter rotation induced by the main rotor to maintain stable flight. The control of the main rotor by the pilot is achieved by a series of control rods that tilt and/or raise and lower a rotor swash plate which controls the angle of attack of the rotor blades. Similarly, the control of the tail rotor by the pilot is achieved by control rods that vary the pitch of the tail rotor blades. A helicopter has three basic separate control inputs and, in addition, a throttle control. The main controls are the Cyclic, the Collective, and the Anti-torque Pedals which control the thrust of the tail rotor. The Cyclic and Collective controls may be linked together so that the push rods, in combination, provide both Cyclic and Collective control of the main rotor.
The Cyclic is usually located between or adjacent to the pilot's legs. It is called the cyclic because it changes the pitch of the main rotor blades depending on their position as they rotate around the hub. The change in the Cyclic pitch has the effect of changing the angle of attack and thus the lift generated by each individual blade as it moves around the rotor swash plate. This in turn causes the blades to change angle of attack up or down in sequence, depending on the tilt of the rotor swash plate. If the rotor swash plate tilts forward, a thrust vector is produced in the forward direction. If the pilot pushes the cyclic stick to the right, the rotor swash plate tilts to the right and produces thrust in that direction, causing the helicopter to move sideways in a hover or to roll into a right turn during forward flight, much as in a fixed wing aircraft. Similarly, direction of flight to the rear and to the left is controlled by moving the Cyclic aft or to the left, respectively.
The Collective pitch control, or Collective, is normally located to the left of the pilot's seat. The Collective changes the pitch angle of all the main rotor blades at the same time independent of their position about the rotor swash plate and results in increases or decreases in total lift. In level flight, this would cause a climb or descent. With the helicopter pitched forward, an increase in total lift would produce an acceleration together with a given amount of ascent.
The Anti-torque Pedals are located in the same position as the rudder pedals in a fixed wing aircraft and serve the same purpose. Pressing the Anti-torque Pedals changes the pitch of the tail rotor blades, increasing or reducing the thrust produced by the tail rotor causing the nose of the helicopter to yaw in the direction of the applied pedal.
For cyclic control, small helicopters depend primarily on tilting the main rotor thrust vector to produce control moments about the aircraft center of gravity (CG), causing the helicopter to roll or pitch in the desired direction. Pushing the Cyclic control forward abruptly from either straight-and-level flight or after a climb can put the helicopter into a low-G flight condition. “Low-G” is sometimes referred to as “weightless” owing to the fact the pilot and passengers may feel a sense of weightlessness most often associated with free fall. “Low-G” is a well understood term in the art and generally refers to the airframe being temporarily partially or totally unloaded. During the low-G condition, lateral Cyclic has little, if any, effect because the rotor thrust has been reduced. In a counter-clockwise rotor system, there is no main rotor thrust component to the left to counteract the tail rotor thrust to the right, and since the tail rotor is above the CG, the tail rotor thrust causes the helicopter tail to rise and the entire aircraft to roll rapidly to the right. In a clock-wise rotor system, the effects are the reverse. If the pilot attempts to stop the right roll by applying full left (or right in a clockwise system) Cyclic before regaining main rotor thrust, the rotor can exceed its flapping limits and cause structural failure of the rotor shaft due to mast bumping, or it may allow a rotor blade to contact the airframe or the pilot may simply lose control.
The Federal Aviation Administration (F.A.A.) regards low-G conditions as being among the more significant flight risks to rotary wing aircraft operators. For this reason, a minimum of two hours of safety training on the subject is generally required before pilots are permitted to operate a Robinson helicopter, for example. The safety training traditionally places higher emphasis on avoidance of low-G conditions as opposed to the corrective maneuvers necessary to recover if a low-G event occurs. As discussed in Safety Notice SN-11 of the Robinson Helicopter Company, even highly experienced and skilled pilots suffer catastrophic/fatal accidents after entering low-G conditions, and thus avoiding low-G scenarios in the first place is viewed as the best approach to accident avoidance.
Despite the well-known severity of low-G conditions, most rotary wing aircraft do not have any system or device for detecting low-G conditions. In such cases, pilots may be advised that a “lightness in the seat of the pants” may be an indication that the aircraft is experiencing a low-G event. This “intuitive” approach to identifying low-G conditions is highly subjective and indefinite and arguably fails to provide any safety measure of real substance.
U.S. Pat. No. 4,763,285 to Moore et al. (“Moore”) discloses a system which identifies low-G conditions in various bands of severity using an accelerometer. The system issues a warning signal to the helicopter pilot in the form of a warbling tone and a suitable warning light. A significant drawback to Moore is a failure to take into account the expected human response for correcting a low-G condition. In most cases, a pilot may have a matter of seconds (e.g., 1-2 seconds) to make a proper corrective maneuver to avoid a catastrophic accident once a low-G event is occurring. Rotary wing aircraft pilots do not frequently encounter low-G conditions and thus, despite the aforementioned safety training, often make a wrong corrective maneuver after becoming aware of low-G conditions per a system such as is disclosed by Moore. An intuitive response of the pilot to a low-G condition is often to apply left lateral Cyclic, the effects of which can be catastrophic. By the time a pilot deduces that he or she has made the wrong correction, an accident may already be occurring or unavoidable. Furthermore, generic auditory alerts such as warbling tones are commonly used for a plurality of purposes such as, for example, an autopilot disconnect. The seconds required for a pilot to identity the precise problem to which a generic auditory alert refers could exhaust the small window of time during which the aircraft is still recoverable from the low-G event. Systems which use visual light warnings such as in Moore are discouraged by the F.A.A., as they introduce a distraction in the cockpit which might only exacerbate the safety hazard or can be misinterpreted since there are other warning lights.
Accordingly, there is a continuing need for systems or devices which reduce the frequency of accidents associated with low-G conditions in rotary wing aircraft.